The immune system has the potential to eliminate altered neoplastic cells with incredible specificity. A consistent in-frame deletion in the extra-cellular domain of the epidermal growth factor receptor (EGFRvIII) represents a truly tumor-specific target amenable to immunotherapeutic attack. Our multi-institutional Phase II study demonstrated that vaccination with an EGFRvIII-specific peptide in patients with newly-diagnosed glioblastoma multiforme (GBM) induces potent T- and B-cell immunity, produces nearly complete radiographic responses in all patients with residual tumor, and universally eliminates EGFRvIII-expressing cells. Recurrent tumors, however, continue to express wild-type EGFR suggesting that the immune response is specific, but productive intra-molecular cross-priming against other potential tumor-associated antigens is incomplete. We believe that productive extension of such secondary immune responses is hindered by the presence of regulatory T-cells (TRegs). We have recently shown that TRegs are disproportionately represented within the peripheral blood and tumors of patients with GBM and serve to induce a state of profound, but reversible, immunosuppression. TRegs are characterized by constitutive expression of the high affinity interleukin (IL)-2 receptor (IL-2R1)(CD25) and are uniquely dependent on IL- 2R1 signaling for their function and survival. Using our spontaneous murine glioma model, we have demonstrated that treatment with an antibody that blocks IL-2R1 signaling functionally inactivates and eliminates TRegs without inducing autoimmune toxicity. Our pre-clinical studies have shown that these unarmed IL-2R1-specific antibodies when given in vivo to mice during recovery from lymphopenia induced by therapeutic temozolomide (TMZ) are capable of not only functionally inactivating TRegs, but also dramatically enhance vaccine-induced immune responses. Daclizumab, an existing, humanized, unarmed IL-2R1-specific antibody, functions identically to the antibody used for TReg inactivation studies in mice. We hypothesize that daclizumab therapy during the recovery from therapeutic TMZ-induced lymphopenia in patients with newly-diagnosed GBM will inhibit the functional recovery of TRegs, enhance immune responses against an EGFRvIII-targeted vaccine, and promote productive cross-priming without the induction of deleterious autoimmunity. Because NK cells also express CD25 and may be potent activators or inhibitors of innate and antigen-specific immune responses, the effect of daclizumab on NK cells will also be assessed.